Water potential in plants is key measurement for determining the water activity in a living cell. Measured water potential in plants is expressed in pressure units, with negative magnitudes. Water potential in plants ranges from ˜−0.3 MPa at the roots, through ˜−1 MPa (Mega-Pascal) in the stem and ˜−2 MPa at the canopy and towards ˜−100 MPa in the atmosphere. As water flow from high potential to low potential, so does water in plants flow from the soil to roots via the stem and the canopy to the atmosphere. Accurate measurement of within plant water potential is essential information for determination of irrigation state of the crop.
Continuous monitoring of water potential in crops is key factor for determination of the optimal irrigation in precision agriculture. The invention described here uses osmometer based sensor to accurately follow the crop water potential.
While connecting measuring devices into xylem vessels disrupts the soil-plant-atmosphere water continuum, the feasibility of measuring apoplastic (inter cellular tissue space) water potentials of abraded stem tissue by psychrometry was demonstrated by McBurney and Costigan (1984), and the instrument was developed by Dixon and Tyree (1984). That approach is commercialized now by ICT International of Australia (Anon. 2013). The rigorous operational requirements of this instrument limit its use to scientific applications, but the device not suitable to practical farming. A novel MEMS device for water potential measurement, was patented by Cornell University under provisional US application Ser. No. 61/170,223, April 2009, PCT publication WO2010/121176 A2 October 2010, entitled “Microfluidic Xylem Probe” and U.S. Pat. No. 8,695,407. This probe is inserted into the stem and measures the water potential via the vapor phase, which is in essence another implementation of a psychrometer. Membrane osmometry is a well-known method, used for many decades in various technical forms (e.g. U.S. Pat. No. 4,455,864 of 1984, UK patent GB32261513 of 1992). It consists of two chambers separated with a semi-permeable membrane: The sealed compartment is filled with known osmotic potential solution, and connected to a pressure sensor; the other chamber contains the fluid with unknown osmotic potential due to molecular weight and/or solute concentration. Since only water molecules can pass through the membrane, water is flowing from high water potential to low. The unknown solution's potential is calculated from the pressure change in the sealed compartment. The invention will use the osmometer principle to measure the stem water potential in the fluid phase by tight contact of the sensor membrane with the stem sap.
U.S. Pat. No. 4,455,864 discloses a membrane osmometer for direct measurement of osmotic pressures. The osmometer includes a pressure measuring chamber for receiving pure solvent, a sample chamber separated therefrom for receiving solution to be tested, and a semi-permeable membrane contacting a support plate coarsely pervious to liquids, the membrane being located on the side of the support plate adjacent to the sample chamber, wherein the sample chamber comprises, on its surface nearest the membrane, a conical annular surface which, when the sample chamber and pressure measuring chamber are assembled, presses an elastic sealing ring against both the surface of the membrane facing the sample chamber and against an inner surface of the pressure measuring chamber
Patent application No. EP0060447 discloses a sample chamber for receiving a sample liquid to be investigated in a membrane osmometer having a sample chamber open to the atmosphere, characterized in that one or more bores or channels are provided in the walls of the chamber, leading from outside into the interior of the sample chamber and opening out in the vicinity of the outer edge of the free portion of the semipermeable membrane.
Patent application No. WO 2004/083829 discloses a membrane osmometer that comprises a semipermeable membrane whose one side is in contact with a solution to be measured that is located in the measuring cell and whose other side is in contact with a solution to be analyzed. A measuring device measures a pressure difference in the measuring cell or a volumetric flow through the membrane. The solution to be measured contains ligands with binding sites and receptors with opposing binding sites whereby enabling the ligands and receptors to form ligand complexes by a binding of the binding sites to the opposing binding sites. The solution to be analyzed contains analytes with a binding site for binding to the opposing binding sites of the receptors. The semipermeable membrane is permeable to analytes and is impermeable to receptors and ligands. During a method for selectively determining a specific analyte in the solution to be analyzed, the analytes diffuse, at least in part, out of the solution to be analyzed and into the measuring cell via the semipermeable membrane. As a result, an equilibrium of ligands, receptors and ligand complexes changes inside the solution to be measured thus effecting a change in pressure inside the measuring cell that can be measured by the measuring device.
Patent application No. WO 2004/083828 discloses a membrane osmometer that is suited for quantitatively determining analytes, which represent low-molecular affinity ligands of a high-molecular affinity receptor, and to a method for quantitatively determining analytes of this type. The inventive method presents an advantageous form of the competitive affinity assay. This method is characterized in that the semipermeable membrane of the inventive membrane osmometer is used as a signal generator and as an interface to the medium phase. Inside the inventive sensory membrane osmometer, the semipermeable membrane is located between a sensory liquid phase and a medium phase. According to the invention, an impermeable affinity receptor and an impermeable competition ligand are located in the sensory liquid phase, and the membrane is permeable to the analytes. According to the invention, the osmotic partial pressure of the impermeable affinity binding partners and or the hydraulic effect of the affinity bonds between the impermeable affinity binding partners and in a network liquid is recorded as a measure of the analyte concentration. To this end, a measuring device is used for measuring the pressure difference over the semipermeable membrane or the volumetric flow through the semipermeable membrane.